Ghrelin O-Acyltransferase (GOAT) is a member of the membrane-bound O-acyl transferase (MBOAT) protein family, and the only enzyme in humans capable of promoting an acylation reaction on the peptide hormone ghrelin. By linking a medium-chain fatty acid to the Serine-3 position of the 28-amino acid peptide, GOAT converts unacylated ghrelin (UAG) to acylated ghrelin (AG) which is the natural ligand of the ghrelin receptor GHSR1a (growth hormone secretagogue receptor 1a). The ghrelin receptor is expressed in various areas of the brain involved in energy homeostasis. Activation of the receptor by AG results in stimulation of neuronal pathways leading to increased food intake, fat deposition and weight gain thus linking the ghrelin system to obesity. In humans, AG in plasma peaks immediately before mealtimes and drops in response to food intake (D. E. Cummings et al., Diabetes (2001) 50(8), 1714-1719). Infusion of AG has been shown to increase food intake in lean and obese subjects (M. R. Druce et al., Int. J. Obes. (2005), 29(9), 1130-1136). So far no receptor has been identified for UAG, but it has been shown to have functional antagonistic effects to AG at least with respect to its metabolic properties (W. Zhang et al., Endocrinology (2008) 149 (9), 4710-4716). Since an inhibitor of GOAT would substantially diminish the level of the GHSR1a ligand AG and concomitantly increase the functional antagonist UAG, it would be useful for the treatment of obesity as an adjunct to a reduced-calorie diet and increased physical activity for chronic weight management.
Insatiable hunger and severe obesity are characteristic features of the Prader-Willi-Syndrome (PWS), a genetically caused orphan disease with a complex pathology. AG levels in plasma of PWS subjects are elevated and AG/UAG ratios are increased suggesting a causal relationship (N. Wierup et al., Regulatory Peptides (2002) 107, 63-69; R. J. Kuppens et al., Endocrine (2015) 50(3), 633-642). Therefore GOAT inhibitors may be effective in reducing food craving behavior and body weight in PWS patients ameliorating one major burden affecting the patients and their families.
Furthermore the ghrelin system seems to play a major role in glucose homeostasis. Administration of AG to human subjects leads to suppression of glucose-induced insulin secretion and an increase in plasma glucose. Infusion of UAG is able to counteract the hyperglycemic effect of AG (F. Broglio et al., J. Clin. Endocrinol. Metab. (2004) 89, 3062-3065). The expression of GOAT, ghrelin and GHSR1a in human pancreatic islets suggests a paracrine role on insulin secretion (A. DelParigi et al., J. Clin. Endocrinol. Metab. (2002) 87(12), 5461-5464). In addition UAG promotes pancreatic β-cell and human islet cell survival in vitro (R. Granata et al., Endocrinology (2007) 148(2), 512-529) and prevents diabetes in streptozotocin treated rats (R. Granata et al., J. Med. Chem. (2012) 55(6), 2585-2596). Thus treatment with a GOAT inhibitor is expected to improve glucose homeostasis in patients with type 2 diabetes or obese with impaired glucose tolerance.